Method of manufacturing nanostructures

ABSTRACT

A method of manufacturing structures includes a stripping step in which an aluminum member that includes an aluminum substrate and an anodized layer present on a surface of the aluminum substrate and that serves as a cathode is electrolyzed to strip the anodized layer from the aluminum substrate to obtain a structure composed of the anodized layer. Electrolysis in the stripping step is carried out in such a way that a current passes over a surface of the anodized layer. Structures having a well-ordered array of pits can be obtained in a short time without the use of substances such as chromic acid that are deleterious to the environment.

The entire contents of all documents cited in this specification areincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a nanostructure and its manufacturingmethod.

In the technical field of metal and semiconductor thin films, wires anddots, it is known that the movement of free electrons becomes confinedat sizes smaller than some characteristic length, as a result of whichsingular electrical, optical and chemical phenomena become observable.Such phenomena are called “quantum mechanical size effects” or simply“quantum size effects.” Functional materials which employ such singularphenomena are under active research and development. Specifically,materials having structures smaller than several hundred nanometers insize, typically called microstructures or nanostructures, are thesubject of current efforts in material development.

Methods for manufacturing such microstructures include processes inwhich a nanostructure is directly manufactured by semiconductorfabrication technology, including micropatterning technology such asphotolithography, electron beam lithography, or x-ray lithography.

Of particular note is the considerable amount of research beingconducted today on processes for manufacturing nanostructures having anordered microstructure.

One method of forming an ordered structure in a self-regulating manneris illustrated by an anodized alumina layer (anodized layer) obtained bysubjecting aluminum to anodizing treatment in an electrolytic solution.It is known that a plurality of micropores having diameters of aboutseveral nanometers to about several hundreds of nanometers are formed ina regular arrangement within the anodized layer. It is also known thatwhen a completely ordered arrangement is obtained by the self-orderingtreatment of this anodized layer, hexagonal columnar cells will betheoretically formed, each cell having a base in the shape of a regularhexagon centered on a micropore, and that the lines connectingneighboring micropores will form equilateral triangles.

For example, H. Masuda et al. (Jpn. J. Appl. Phys., Vol. 37, Part 2, No.11A, pp. L1340-1342 (Nov. 1, 1998), FIG. 2) describes an anodized layerhaving micropores whose pore size dispersion is 3% or less. In anotherrelated publication (Hyomen Gijutsu Binran [Handbook of SurfaceTechnology], edited by The Surface Finishing Society of Japan (NikkanKogyo Shimbun Co., Ltd., 1998), pp. 490-553), it is described thatmicropores are naturally formed in an anodized layer as oxidationproceeds. Moreover, H. Masuda (“Highly ordered metal nanohole arraybased on anodized alumina”, Kotai Butsuri [Solid State Physics], Vol.31, No. 5, pp. 493-499 (1996)) has proposed the formation of a gold dotarray on a silicon substrate using a porous anodized layer as the mask.

A plurality of micropores take on a honeycomb-like structure in whichthe micropores are formed parallel in a direction substantially verticalto the substrate surface, and at substantially equal intervals. Thispoint is deemed to be the most distinctive characteristic of anodizedlayers in terms of material. Another remarkable feature of anodizedlayers, thought to be absent in other materials, is the ability torelatively freely control the pore diameter, pore spacing and pore depth(see Masuda, 1996).

Known examples of applications for anodized layers include various typesof devices, such as nanodevices, magnetic devices, and luminescentdevices. For example, JP 2000-31462 A mentions a number of applications,including magnetic devices in which the micropores are filled with themagnetic metal cobalt or nickel, luminescent devices in which themicropores are filled with the luminescent material ZnO, and biosensorsin which the micropores are filled with enzymes/antibodies.

In addition, in the field of biosensing, JP 2003-268592 A describes anexample in which a structure obtained by filling the interior ofmicropores in an anodized layer with a metal is used as a sample holderfor Raman spectroscopy.

Raman scattering is the effect where, when incident light (photons)strikes particles and scatters, inelastic collisions with the particlesarise, causing a change in energy. Raman scattering is used as atechnique for spectroscopic analysis, but a current challenge is how toenhance the intensity of the scattered light used in measurement so asto improve the sensitivity and accuracy of analysis.

A phenomenon that enhances Raman scattered light is known as thesurface-enhanced resonance Raman scattering (SERRS) effect. This effectis one where the scattering of certain kinds of molecules absorbed ontothe surface of, for example, a metal electrode, a sol, a crystal, avapor-deposited film or a semiconductor, is enhanced relative to withina solution. A remarkable enhancement effect of from 10¹¹ to 10¹⁴ timesis seen particularly with gold and silver. The mechanism underlying theSERRS effect is not yet fully understood, although the surface plasmonresonance described below is believed to play a role. Use of the plasmonresonance principle as a means for enhancing the Raman scatteringintensity is a stated object in JP 2003-268592 A as well.

Plasmon resonance is the effect where, when the surface of a noble metalsuch as gold or silver is irradiated with light so that the metalsurface is placed in an excited state, plasmon waves—which are localizedelectron density waves, interact with electromagnetic waves (resonanceexcitation) to form a resonance state. Surface plasmon resonance (SPR)is a type of plasmon resonance in which, when the metal surface isirradiated with light, free electrons at the metal surface acquire anexcited state and collectively oscillate, generating a surface plasmonwave which in turn generates a strong electric field.

In the near-surface region where plasmon resonance arises, that is, inthe region within about 200 nm from the surface, an electric fieldenhancement of several decades (e.g., 10⁸ to 10¹⁰ times) can be seen,and a distinct rise is observed in various optical effects. For example,when light is directed at a prism having thereon a vapor-deposited thinfilm of a suitable metal such as gold at an angle larger than thecritical angle, changes in the dielectric constant of the thin-filmsurface can be detected to a high sensitivity as changes in theintensity of the reflected light due to the surface plasmon resonanceeffect.

Specifically, using a SPR sensor which employs the surface plasmonresonance effect, quantitative measurement of reactions and bondsbetween biomolecules and kinetic analysis can be carried out withoutlabeling and in real time. SPR sensors are used in research on immuneresponse, signal transduction, and interactions between varioussubstances such as proteins and nucleic acids. Recently, a paper waseven published on analyzing trace dioxins using an SPR sensor (Karube,et al., Analytica Chimica Acta 434, No. 2, 223-230 (2001)).

Various methods are being studied for increasing plasmon resonance,including techniques that involve localizing plasmons by using the metalin the form of discrete particles rather than as a thin film. Forexample, JP 2003-268592 A describes a technique in which localization isinduced by providing metal particles on well-ordered pores in ananodized layer.

According to a research article, when localized plasmon resonance withmetal particles is used, if the metal particles are present in closeproximity to each other, the electric field strength is enhanced in thegaps between the metal particles, thereby achieving a state that makesit easier to generate a plasmon resonance (see T. Okamoto: “A study onmetal nanoparticle interactions and biosensors”, found in an Internetsearch on Nov. 27, 2003 at http://www.plasmon.jp/reports/okamoto.pdf).

In processes which use the self-ordering treatment of an anodized layerto fabricate an anodized layer having a well-ordered arrangement ofmicropores thereon, it has hitherto been customary to carry out aself-ordering step in which electrolysis is carried out for an extendedperiod of time under specific electrolytic conditions so as to promotethe orderly formation of micropores, then to carry out a layer removalstep in which the anodized layer obtained in the self-ordering step isdissolved in a mixed aqueous solution of chromic acid and phosphoricacid so that the bottom portion of the micropores where the pores arethe most regularly arrayed is revealed at the surface.

JP 61-88495 A describes a process for obtaining a porous layer byperforming anodizing treatment on an aluminum member or an aluminumalloy so as to form a porous layer, then using reverse electrolysismeans to strip just the porous layer from the parent material.

An article in Aruminium Kenkyukaishi (Vol. 201, No. 7, pp. 7-8 (1985))describes a process in which the barrier layer is thinned by an electriccurrent recovery technique, following which reverse electrolysis meansis used to strip the anodized layer from the aluminum member.

SUMMARY OF THE INVENTION

However, in spite of differences due to the thickness of the anodizedlayer, it is generally necessary for the layer removal step which uses amixed aqueous solution of chromic acid and phosphoric acid to be carriedout over a long period of time ranging from several hours to well overten hours. Also, the anodized layer is dissolved, making effective useof this layer impossible. Moreover, such a process has required the useof chromic acid, which is a substance that is bad for the environment.

When use is made of a process that involves reducing the film thicknessby an electric current recovery method, then employing reverseelectrolysis means to strip the anodized layer from the aluminum member,as described in JP 61-88495 A and Aruminium Kenkyukaishi (Vol. 201, No.7, pp. 7-8 (1985)), it was found that the electric current recoverymethod leads to the formation of very small branched pores at the bottomof the micropores, resulting in the disruption of regularly arrayed pitsat the surface of the aluminum member obtained by delamination.Therefore, even when an anodized layer is formed by additionallyperforming anodizing treatment on the resulting aluminum member, use inapplications such as a sample holder for Raman spectroscopy has not beenpossible.

It is therefore an object of the invention to provide a manufacturingmethod from which structures having a well-ordered array of pits can beobtained in a short time without the use of substances such as chromicacid that are deleterious to the environment. Another object of theinvention is to provide structures obtained by such manufacturingmethod.

The inventors have made intensive studies to achieve the above objectsand found that by performing electrolysis using as the cathode analuminum member having an anodized layer so that the current passes onlyover the surface of the anodized layer, a structure having awell-ordered array of pits can be obtained in a short period of time.

Accordingly, the invention provides the following (1) to (11).

(1) A method of manufacturing a structure comprising:

a stripping step in which an aluminum member that includes an aluminumsubstrate and an anodized layer present on a surface of the aluminumsubstrate and that serves as a cathode is electrolyzed to strip theanodized layer from the aluminum substrate to thereby obtain a structurecomposed of the anodized layer,

wherein electrolysis in the stripping step is carried out in such a waythat a current passes over a surface of the anodized layer.

(2) The method of manufacturing the structure according to (1) above,wherein the electrolysis in the stripping step is carried out in such away that the current passes only over the surface of the anodized layer.

(3) The method of manufacturing the structure according to (2) above,wherein the electrolysis in the stripping step is carried out in a statewhere an electrolytic solution is in contact with the surface of theanodized layer but is in contact with neither the aluminum substrate noredges of the anodized layer.

(4) The method of manufacturing the structure according to any one of(1) to (3) above, wherein the electrolysis in the stripping step reachescompletion when the current falls to a value of 0.1 A/dm² or below.

(5) A structure obtained by the method according to any one of (1) to(4) above.

(6) A method of manufacturing a structure comprising:

a stripping step in which an aluminum member that includes an aluminumsubstrate and an anodized layer present on a surface of the aluminumsubstrate and that serves as a cathode is electrolyzed to strip theanodized layer from the aluminum substrate to thereby obtain a structurecomposed of the aluminum substrate having pits formed therein; and

an anodizing step in which the aluminum substrate having the pits formedtherein are anodized to obtain the structure composed of the aluminumsubstrate having on a surface thereof a micropore-bearing anodizedlayer,

wherein electrolysis in the stripping step is carried out in such a waythat a current passes over a surface of the anodized layer.

(7) The method of manufacturing the structure according to (6) above,wherein the electrolysis in the stripping step is carried out in such away that the current passes only over the surface of the anodized layer.

(8) The method of manufacturing the structure according to (7) above,wherein the electrolysis in the stripping step is carried out in a statewhere an electrolytic solution is in contact with the surface of theanodized layer but is in contact with neither the aluminum substrate noredges of the anodized layer.

(9) The method of manufacturing the structure according to any one of(6) to (8) above, wherein the electrolysis in the stripping step reachescompletion when the current falls to a value of 0.1 A/dm² or below.

(10) The method of manufacturing the structure according to any one of(6) to (9), further comprising a chemical dissolution treatment stepwhich follows the anodizing step and in which the structure composed ofthe aluminum substrate having on the surface thereof themicropore-bearing anodized layer is subjected to a chemical dissolutiontreatment.

(11) A structure obtained by the method according to any one of (6) to(10) above.

The manufacturing method of the invention enables structures havingwell-ordered arrays of pits to be obtained in a short period of time.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a graph of the changes in electrical current over time whenelectrolysis is performed, using an aluminum member having an aluminumsubstrate and an anodized layer as the cathode, in such a way that thecurrent passes only over the surface of the anodized layer;

FIGS. 2A to 2D show diagrams illustrating the inventive method ofmanufacturing structures;

FIG. 3 shows a schematic diagram of a special jig that may be used inreverse electrolysis; and

FIGS. 4A and 4B show diagrams illustrating a method for computing thedegree of ordering of pores.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described more fully below.

The first aspect of the invention provides a method of manufacturing astructure including a stripping step in which an aluminum member thatincludes an aluminum substrate and an anodized layer present on asurface of the aluminum substrate and that serves as a cathode iselectrolyzed to strip the anodized layer from the aluminum substrate tothereby obtain a structure composed of the anodized layer. In thestripping step, electrolysis is carried out in such a way that a currentpasses only over a surface of the anodized layer.

<Aluminum Member>

The aluminum member used in the invention has the aluminum substrate andthe anodized layer present on the surface of the aluminum substrate.Such an aluminum member may be obtained by performing anodizingtreatment on the surface of the aluminum substrate.

<Aluminum Substrate>

The aluminum substrate is not subject to any particular limitation.Illustrative examples include commercial aluminum substrates; substratesmade of low-purity aluminum (e.g., recycled material) on whichhigh-purity aluminum has been vapor-deposited; substrates such assilicon wafers, quartz or glass whose surface has been covered withhigh-purity aluminum by a process such as vapor deposition orsputtering; and resin substrates on which aluminum has been laminated.

Of the aluminum substrate, the surface on which an anodized layer isprovided by anodizing treatment has an aluminum purity of preferably atleast 99.5 wt %, and more preferably at least 99.80 wt %, but preferablyless than 99.99 wt %, and more preferably 99.95 wt % or less. At analuminum purity of 99.5 wt % or more, the pore arrangement will besufficiently well-ordered, and at an aluminum purity of less than 99.99wt %, inexpensive production is possible.

It is preferable for the surface of the aluminum substrate to besubjected beforehand to degreasing and mirror-like finishing treatment.

<Degreasing>

Degreasing is carried out with a suitable substance such as an acid,alkali or organic solvent so as to dissolve and remove organicsubstances (primarily oils) adhering to the surface. Known degreasersmay be used in degreasing treatment.

For example, degreasing may be carried out using any of variouscommercially available degreasers by the prescribed method.

Degreasing may be carried out by, for example, immersing the aluminumsubstrate for a length of time during which only a small amount of airbubbles evolve from the aluminum surface in an aqueous solution ofsodium hydroxide having a pH of 10 to 13 and a temperature of about 30°C. to about 50° C. or in an aqueous sulfuric acid solution having a pHof 1 to 4 and a temperature of about 40° C. to about 70° C.

Preferred degreasing treatment is exemplified by washing the aluminumsubstrate with acetone, then immersing the substrate in sulfuric acidhaving a pH of 4 and a temperature of 50° C. This method is advantageousbecause it removes oils on the aluminum surface without substantiallyany dissolution of the aluminum.

<Mirror-Like Finishing>

Mirror-like finishing is carried out to eliminate surface asperities onthe aluminum substrate and improve the uniformity and reproducibility ofsealing treatment by a process such as electrodeposition.

In the practice of the invention, mirror-like finishing is not subjectto any particular limitation, and may be carried out using any suitablemethod known in the art. Illustrative examples of suitable methodsinclude polishing with various commercial abrasive cloths, methods thatcombine the use of various commercial abrasives (e.g., diamond, alumina)with buffing, electrolytic polishing and chemical polishing. Thesemethods may be used in appropriate combinations.

Examples of electrolytic polishing and chemical polishing methodsinclude various methods mentioned in the 6^(th) edition of AluminumHandbook (Japan Aluminum Association, 2001), pp. 164-165.

Mirror-like finishing is preferably performed by a method in whichpolishing is performed using abrasives while changing over time theabrasive used from one having coarser particles to one having finerparticles and thereafter electrolytic polishing is performed. In such acase, the final abrasive used is preferably one having a grit size of1500. This method is capable of removing rolling streaks that may beformed during rolling when the aluminum substrate has been produced by aprocess including rolling.

Mirror-like finishing enables a surface having, for example, an averagesurface roughness R_(a) of 0.03 μm or less and a glossiness of at least70% to be obtained. The average surface roughness R_(a) is preferably0.02 μm or less. The glossiness is preferably at least 80%.

The glossiness is the specular reflectance which can be determined inaccordance with JIS Z8741-1997 (Method 3: 60° Specular Gloss) in adirection perpendicular to the rolling direction.

<Anodizing Treatment>

Any conventionally known method can be used for anodizing treatment.More specifically, a self-ordering method to be described below ispreferably used.

The self-ordering method is a method which enhances the orderliness byusing the regularly arranging nature of micropores in the anodized layerand eliminating factors that may disturb an orderly arrangement.Specifically, an anodized layer is formed on high-purity aluminum at avoltage appropriate for the type of electrolytic solution and at a lowspeed over an extended period of time (e.g., from several hours to wellover ten hours).

Typical examples of self-ordering methods include those described in J.Electrochem. Soc. Vol. 144, No. 5, p. L128 (May 1997); Jpn. J. Appl.Phys. Vol. 35, Part 2, No. 1B, p. L126 (1996); Appl. Phys. Lett. Vol.71, No. 19, p. 2771 (Nov. 10, 1997), and in the above-referenced articleby Masuda (1998).

Use of high-purity materials and treatment performed at a relatively lowtemperature for a long period of time at a specified voltage determinedaccording to the electrolytic solution are the technical features of themethods described in these well-known articles. More specifically, thesemethods each use a material having an aluminum purity of at least 99.99wt % to carry out the self-ordering method under the conditionsindicated below.

-   (1) 0.3 mol/L sulfuric acid, 0° C., 27 V, 450 minutes (J.    Electrochem. Soc., 1997)-   (2) 0.3 mol/L sulfuric acid, 10° C., 25 V, 750 minutes (J.    Electrochem. Soc., 1997)-   (3) 0.3 mol/L oxalic acid, 17° C., 40 to 60 V, 600 minutes (Jpn. J.    Appl. Phys., 1996)-   (4) 0.04 mol/L oxalic acid, 3° C., 80 V, layer thickness, 3 μm    (Appl. Phys. Lett., 1997)-   (5) 0.3 mol/L phosphoric acid, 0° C., 195 V, 960 minutes (Appl.    Phys. Lett., 1997).

The self-ordering anodizing treatment used in this invention may becarried out by, for example, a method that involves passing anelectrical current through the aluminum substrate as the anode in asolution having an acid concentration of 1 to 10 wt %. Solutions thatmay used in anodizing treatment include any one or combinations of twoor more of the following: oxalic acid, sulfuric acid, citric acid,malonic acid, tartaric acid and phosphoric acid.

The conditions of the self-ordering anodizing treatment vary dependingon the electrolytic solution used, and thus cannot be strictlyspecified. However, it is generally suitable for the electrolyteconcentration to be 0.01 to 10 mol/L, the temperature of the solution tobe 0 to 20° C., the current density to be 0.1 to 10 A/dm², the voltageto be 15 to 240 V, the amount of electricity to be 3 to 10,000 C/dm²,and the period of electrolysis to be 30 to 1,000 minutes.

As for the electrolysis, potentiostatic electrolysis is preferablyperformed.

The anodized layer has the following properties.

The thickness, including the barrier layer, is preferably at least 0.1μm, and more preferably at least 1 μm. Within this range, the microporesare even more highly ordered.

Moreover, the thickness, including the barrier layer, is preferably notmore than 100 μm. Within this range, stripping from the aluminumsubstrate in the subsequently described stripping step is easy.

The barrier layer has a thickness of preferably not more than 600 nm,more preferably from 5 to 400 nm, and even more preferably from 10 to 80nm. Within this range, the strippability in the subsequently describedstripping step is excellent.

The pore diameter is from 10 to 500 nm, preferably from 15 to 100 nm,and more preferably from 20 to 80 nm. Within this range, when themicropores are filled with metal, the micropores are more uniformlyfilled with the metal.

The pore diameter has a coefficient of variation which, while notsubject to any particular limitation, is preferably less than 30%, andmore preferably from 5 to 20%.

The coefficient of variation (CV) of the pore diameter is an indicatorof the variation in the pore size. It is defined by the followingequation.Coefficient of Variation of Pore Diameter=(standard deviation of porediameter)/(average pore diameter)

The micropores have a period of preferably from 20 to 700 nm, morepreferably from 25 to 600 nm, and even more preferably from 25 to 150nm.

The period of the micropores has a coefficient of variation which, whilenot subject to any particular limitation, is preferably less than 30%,and more preferably at least 5% but less than 20%.

The area ratio occupied by the micropores is preferably from 10 to 70%.

<Pore Widening Treatment>

In the practice of the invention, the anodized layer of theabove-described aluminum member may be subjected to pore wideningtreatment.

Pore widening treatment, which is carried out after anodizing treatment,is performed by immersing the aluminum substrate in an aqueous solutionof an acid or an alkali so as to dissolve the anodized layer and enlargethe diameter of the micropores. This makes it easy to control theregularity of the micropore array.

When pore widening treatment is carried out with an aqueous acidsolution, it is preferable to use an aqueous solution of an inorganicacid such as sulfuric acid, phosphoric acid, nitric acid or hydrochloricacid, or a mixture thereof. It is desirable for the aqueous acidsolution to have a concentration of 1 to 10 wt % and a temperature of 25to 40° C.

When pore widening treatment is carried out with an aqueous alkalisolution, it is preferable to use an aqueous solution of at least onealkali selected from the group consisting of sodium hydroxide, potassiumhydroxide and lithium hydroxide. It is preferable for the aqueous alkalisolution to have a concentration of 0.1 to 5 wt % and a temperature of20 to 35° C.

Specific examples of preferred solutions include a 40° C. aqueoussolution containing 50 g/L of phosphoric acid, a 30° C. aqueous solutioncontaining 0.5 g/L of sodium hydroxide, and a 30° C. aqueous solutioncontaining 0.5 g/L of potassium hydroxide.

The immersion time in the aqueous acid solution or aqueous alkalisolution is preferably 8 to 60 minutes, more preferably 10 to 50minutes, and even more preferably 15 to 30 minutes.

<Barrier Layer Thinning Treatment>

In one preferred embodiment of the invention, the above-describedaluminum member is an aluminum member in which the barrier layer of theanodized layer has been thinned. When the barrier layer has beenthinned, the strippability in the subsequently described stripping stepis excellent.

The inventors have found that, by using a method which gradually lowersrather than suddenly changing the voltage, i.e., a method which lowersthe voltage after anodizing treatment without generating a currentrecovery period while maintaining a state of constant current flow, thebarrier layer of the anodized layer can be thinned without a loss in theregularity of the arrangement of micropores on the anodized layer. Thisis presumably because fine branching does not arise owing to the factthat a current recovery period is not generated.

Specifically, when the voltage of anodizing treatment is, for example,100 V or more, the voltage drop rate is preferably set to 20 V/min orless, more preferably 10 V/min or less, and even more preferably 5V/minor less.

The higher the current maintained, the better. Specifically, the currentis preferably at least 10 μA/cm², more preferably at least 30 μA/cm²,and even more preferably at least 50 μA/cm².

If the current is too low, the regularity of the micropore array isdisrupted. Therefore, when the current falls to below 10 μA/cm² at theabove-indicated rate, it is preferable to stop the voltage drop andawait a current flow of at least 10 μA/cm² before continuing the voltagedrop.

<Other Treatment>

Other treatments may be performed as needed.

For example, when the structure of the invention is to be used as asample holder on which an aqueous solution will be deposited to form afilm, hydrophilizing treatment may be performed to reduce the contactangle with water. Such hydrophilizing treatment may be performed by amethod known in the art.

Alternatively, when the inventive structure is to be used as a sampleholder for protein that will be denatured or decomposed with acid,neutralizing treatment may be performed to neutralize the acids that areused in anodizing treatment and remain as residues on the aluminumsurface. Such neutralizing treatment may be performed by a method knownin the art.

<Stripping Step>

The stripping step is an operation in which the anodized layer and thealuminum substrate are separated from each other by electrolysis usingthe above-described aluminum member as the cathode to give a structurecomposed of the anodized layer. In the stripping step, electrolysis iscarried out using the aluminum member as the cathode. Because this isthe reverse of electrolysis in anodizing treatment using the aluminummember as the anode, it is referred to below as “reverse electrolysis.”

In the stripping step, such reverse electrolysis generates hydrogen atthe boundary between the anodized layer and the aluminum substrate inthe aluminum member. Apparently, part of that portion of the barrierlayer, which belongs to the anodized layer and lies at the boundarybetween the anodized layer and the aluminum substrate, in contact withthe aluminum substrate is reduced and dissolved by the hydrogen,becoming aluminum ions, causing the anodized layer and the aluminumsubstrate to separate at the boundary therebetween.

In reverse electrolysis, electrolysis is performed by using theabove-described aluminum member as the cathode and passing current alongthe surface of the anodized layer. This facilitates stripping.

In particular, it is preferable to perform electrolysis so that theelectrical current passes only over the surface of the anodized layer.That is, it is preferable to perform electrolysis so that the currentpasses over the surface of the anodized layer and does not pass throughthe aluminum substrate of the aluminum member.

Specifically, electrolysis is performed in a state where, for example,the electrolytic solution is in contact with the surface of the anodizedlayer, but is in contact with neither the edges of the anodized layernor the aluminum substrate. The method for achieving this state is notsubject to any particular limitation. Illustrative examples includemethods in which only the surface of the anodized layer is left exposedby masking or the use of a special jig, following which the aluminummember is immersed in an electrolytic solution; and methods in which theelectrolytic solution is supplied only to the surface of the anodizedlayer.

Methods that involve masking are carried out by using an insulatingmaterial to cover those portions of the aluminum member other than thesurface of the anodized layer which is brought into contact with theelectrolytic solution.

The insulating material, while not subject to any particular limitation,is preferably a material having a volume resistivity at 20° C. of atleast 10¹⁶ Ω·m. Illustrative examples include resins (natural resins andsynthetic resins), rubbers, ceramics (e.g., glass), metal oxides andmica.

Of these, a flexible synthetic resin is preferred. Examples of suchsynthetic resins include polyvinyl chloride, polycarbonate, acrylicresins, PET, epoxy resins, polyimides, polypropylene, polyesters,polyethylene, saran and polyvinylidene chloride.

Preferred use can also be made of insulating tape composed of a backingthat is made of such a synthetic resin and is coated with apressure-sensitive adhesive (such tape is referred to below as “adhesivetape”). Examples of adhesive tapes include epoxy films (e.g., Super 10,produced by the 3M Company), polyimide films (e.g., 1205, produced bythe 3M Company), PTFE films (e.g., 62, produced by the 3M Company),polyester films (e.g., 56, produced by the 3M Company), plastic filmtapes (e.g., Scotch electroplating tape 470, produced by the 3MCompany), polyester films (e.g., ELEP masking tape N-300, produced byNitto Denko Corporation), and polypropylene tapes (e.g., DANPRON,produced by Nitto Denko Corporation).

No particular limitation is imposed on the method of covering with theinsulating material. Illustrative examples include methods that involvecoating a liquid resin (e.g., an adhesive) and methods that involveaffixing adhesive tape. For example, depending on the areas to becovered, these methods may be used in combination, such as by affixingadhesive tape to the back of the aluminum substrate, then coating thealuminum substrate and the edges of the anodized layer with a liquidresin.

Of these, the method of affixing a resin tape to which apressure-sensitive adhesive has been applied is preferable from thestandpoint of the insulating properties within the electrolytic solutionand the ready availability of such tape.

The thickness of the insulating material in the covered areas ispreferably at least 1 μm from the standpoint of electrical insulatingproperties, and preferably from 10 to 200 μm from the standpoint ofhandleability.

When a pressure-sensitive adhesive or an adhesive is applied to theinsulating material, for good coating uniformity and to preventdeformation after bonding, it is preferable that the coating thicknessbe from 10 to 100 μm.

As mentioned above, masking is carried out by covering with aninsulating material the areas of the aluminum member other than thesurface of the anodized layer to be brought into contact with theelectrolytic solution. Specifically, the edges of the anodized layer andthe entire aluminum substrate are covered with the insulating material.

It is also possible to cover just part of the anodized layer. In such acase, it is preferable that a portion of the anodized layer be covered,and that the remaining open area be circular, elliptical or in arectangular shape with rounded corners, as this facilitates stripping. Acircular shape is especially preferred because reverse electrolysis canbe carried out uniformly in all parts of the region where it is carriedout.

Methods that involve the use of a special jig are not subject to anyparticular limitation, provided the method is one which uses a jig thatallows only the surface of the anodized layer to be exposed.

Anodes which may be used in reverse electrolysis are not subject to anyparticular limitation. Illustrative examples include platinum-platedtitanium electrodes, platinum electrodes, and carbon electrodes.

The electrolytic solution used in reverse electrolysis, while notsubject to any particular limitation, is preferably an aqueous solutionof an acid.

The aqueous acid solution has a pH of preferably 1 to 7, more preferably2 to 6, and even more preferably 2.5 to 5.5. The aqueous acid solutionhas an electrical conductivity of preferably from 0.01 to 100 mS/cm, andmore preferably from 0.1 to 50 mS/cm.

When the aqueous acid solution has a pH and an electrical conductivityin the above ranges, good stripping is achieved without corrosion of thealuminum substrate or incomplete removal of the anodized layer.

If the electrical conductivity of the aqueous acid solution is too low,very small current values may not arise. In such a case, it ispreferable to make the ionic concentration in the aqueous acid solutionhigh so as to allow very small current values to arise. On the otherhand, if the ionic concentration in the aqueous acid solution is toohigh, very small current values do arise, but electrolysis ends in ashort time, after which the current value rises rapidly, making controldifficult. Moreover, on exceeding the time in which a very small currentvalue is reached, corrosion occurs.

Preferred examples of acids that may be used in the aqueous acidsolution include oxalic acid, sulfuric acid and phosphoric acid.

Alternatively, use can be made of, for example, metallic salt compoundswhich exhibit acidity when dissolved in water, and organic compoundswhich exhibit acidity when dissolved in water.

Illustrative examples of metallic salt compounds which exhibit aciditywhen dissolved in water include aluminum oxalate, aluminum sulfate,aluminum lactate, aluminum fluoride and aluminum borate.

Preferred organic compounds which exhibit acidity when dissolved inwater are carboxylic acids. Suitable examples include saturatedaliphatic dicarboxylic acids such as adipic acid, unsaturated aliphaticdicarboxylic acids such as maleic acid, aromatic monocarboxylic acidssuch as benzoic acid, aromatic dicarboxylic acids such as phthalic acid,and aromatic oxycarboxylic acids such as salicylic acid.

Alternatively, use can be made of salts which exhibit neutrality whendissolved in water, i.e., neutral salts. Suitable examples of neutralsalts include carbonates such as ammonium carbonate and borates such asammonium borate.

In cases where a neutral salt is used, one preferred embodiment is toprepare a mixed bath which also includes an additive such as a fluoride,a carbonic acid derivative or an acid amide. The fluoride is exemplifiedby ammonium fluoride. The carbonic acid derivative is exemplified byguanidine carbonate, urea and formaldehyde. The acid amide isexemplified by acetamide.

Of these, the use of oxalic acid, aluminum oxalate, sulfuric acid,aluminum sulfate or a mixture thereof is preferred. Aluminum sulfate andsulfuric acid are especially preferred from the standpoint ofavailability and wastewater treatability.

A preferred embodiment involves the use of an electrolytic solution ofthe same type as that used in the above-described anodizing treatment.

The reverse electrolysis conditions vary depending on the electrolyticsolution used, and thus cannot be strictly specified.

When the electrolytic solution is an aqueous solution of oxalic acid,the concentration is preferably from 0.4 to 10 wt %; when it is anaqueous solution of sulfuric acid, the concentration is preferably from2 to 20 wt %; and when it is an aqueous solution of phosphoric acid, theconcentration is preferably from 0.4 to 5 wt %.

The electrolytic solution generally has a temperature of from 0 to 50°C., and preferably from 10 to 35° C.

The current density is preferably from 1 to 400 mA/dm², more preferablyfrom 5 to 400 mA/dm², and even more preferably from 10 to 300 mA/dm². Inthe above range, stripping can be carried out more uniformly.

The voltage is preferably from 5 to 350 V, more preferably from 8 to 300V, and even more preferably from 10 to 240 V. The voltage is preferablylower than the electrolysis voltage applied when forming the anodizedlayer.

In one preferred embodiment, the voltage is held constant. In anotherpreferred embodiment, the voltage is increased over time.

In the practice of the invention, it is preferable to end electrolysisafter the current value has fallen to 0.1 A/dm² or below.

FIG. 1 is a graph of the changes in electrical current over time whenelectrolysis is performed, using an aluminum member having an aluminumsubstrate and an anodized layer as the cathode, in such a way that thecurrent passes only over the surface of the anodized layer.

From immediately after the start of electrolysis (T₁) until T₃, there isa time (T₂) at which the current value reaches a maximum. Because gasevolution is observed in the interval between T₁ and T₃, the hydrogenions H⁺ generated at the boundary between the barrier layer of theanodized layer and the aluminum substrate presumably become hydrogenmolecules as a result of electrochemical reactions.

The current subsequently undergoes an abrupt drop at time T₄. At thistime, it is thought that separation between the anodized layer and thealuminum substrate has been completed. The current value then remainslow until T₅. During the interval between T₄ and T₅, it is assumed thathydrogen gas accumulates between the anodized layer and the aluminumsubstrate.

When reverse electrolysis is continued further, the current valuereaches a peak at T₆. This is thought to be due to the formation ofcracks in the anodized layer.

Next, after T₇, the electrolytic solution presumably penetrates betweenthe anodized layer and the aluminum substrate via the cracks in theanodized layer, causing corrosion to proceed.

In the practice of the invention, it is preferable to end electrolysisduring the interval between T₄ and T₅. In this way, corrosion due tocracking of the anodized layer and penetration of the electrolyticsolution can be prevented. To this end, it is desirable to monitor thecurrent value and to end electrolysis once the current value falls to0.1 A/dm² or below.

The current value in the T₄T₅ interval is typically not more than 30%,and generally not more than 10%, of the maximum value (at time T₂).Therefore, in one preferred embodiment, the current value is monitoredand electrolysis is brought to completion once the current value fallsto not more than 30%, or to not more than 10%, of the maximum currentvalue.

Because metallic aluminum is exposed on the back and edges of thealuminum plate, if masking or the like is not carried out, the currentwill concentrate in the metallic aluminum and will not readily pass overthe anodized layer, making stripping difficult to carry out and thusresulting in a poor stripping uniformity.

Moreover, in the absence of masking or the like, most of the currentpasses through the metallic aluminum, making it difficult to gauge thestripped state of the anodized layer from changes in the current value.As noted above, by covering the edges of the anodized layer and theentire aluminum substrate with an insulating material, the strippedstate of the anodized layer can be understood from changes in thecurrent values.

Following the end of electrolysis, the stripped anodized layer can beseparated from the aluminum substrate by, for example, cutting along theboundary between areas where current was passed and areas where currentwas not passed. To prevent the anodized layer from breaking up due tostress during such cutting, the anodized layer can be removed afterbeing secured to adhesive tape or the like.

There will be times where the aluminum substrate obtained from thestripping step has remaining thereon a remnant of the anodized layer ina thickness of up to 0.2 μm over up to 10% of the stripped surface. Tomake use of such an aluminum substrate, it is desirable that it be freeof any remnants of the anodized layer.

Accordingly, in such a case, it is preferable to remove remnants of theanodized layer by carrying out chemical treatment following reverseelectrolysis. Specifically, removal can be effected by chemicaltreatment (chemical polishing treatment) using a method which involvesbringing any of various acidic or alkaline aqueous solutions intocontact with the anodized layer. The chemical treatment (chemicalpolishing treatment) method is not subject to any particular limitation,and may be carried out by a method known in the art.

Examples of acidic aqueous solutions include aqueous solutions ofphosphoric acid, aqueous solutions of sulfuric acid, aqueous solutionsof nitric acid, aqueous solutions of oxalic acid, and mixed aqueoussolutions of chromic acid and phosphoric acid. Of these, mixed aqueoussolutions of chromic acid and phosphoric acid are preferred.

The acidic aqueous solution has a pH of preferably −0.3 to 6, morepreferably 0 to 4, and even more preferably 2 to 4.

The temperature of the acidic aqueous solution is preferably from 20 to60° C., and more preferably from 30 to 50° C.

The treatment time is preferably from 1 second to 6 hours, morepreferably from 5 seconds to 3 hours, and even more preferably from 10seconds to 1 hour.

Examples of alkaline aqueous solutions include aqueous solutions ofsodium hydroxide, aqueous solutions of sodium carbonate and aqueoussolutions of potassium hydroxide.

The alkaline aqueous solution has a pH of preferably 10 to 13.5, andmore preferably 11 to 13.

The temperature of the alkaline aqueous solution is preferably from 10to 50° C., and more preferably from 20 to 40° C.

The treatment time is preferably from 1 second to 10 minutes, morepreferably from 2 seconds to 1 minute, and more preferably from 3 to 30seconds.

It is also possible to use these methods in combination. One suchexample is a method that involves alkali treatment in which the surfaceof the aluminum substrate is minimally dissolved with an alkalineaqueous solution to remove remnants of the anodized layer, followingwhich desmutting treatment is carried out in which neutralizationproduct that has formed as a result of such alkali treatment isdissolved and removed with an acidic aqueous solution.

A specific illustration would be a process that involves alkalitreatment in which the aluminum substrate is brought into contact withan aqueous solution containing 5 wt % of sodium hydroxide (temperature,70° C.) for 10 seconds, followed by desmutting treatment in which thealuminum substrate is brought into contact with an aqueous solutioncontaining 30 wt % of sulfuric acid (temperature, 50° C.) for 60seconds.

Additional examples are various methods described for chemicalpretreatment in Aruminiumu Gijutsu Binran [Handbook of AluminumTechnology], edited by the Light Metal Association (Kallos PublishingCo., 1996), pp. 926-929. Of these, preferred examples include alkalidegreasing, acid degreasing, electrolytic degreasing, and combinationsthereof, which have an aluminum substrate surface layer dissolvingaction; and alkali etching treatment, acid etching treatment, as well ascombinations thereof, which have a strong aluminum substrate surfacelayer dissolving action.

Moreover, if a portion of the anodized layer remains behind even afterthe stripping step has been carried out, the residual anodized layer canbe completely removed by alternately carrying out the anodizingtreatment and the stripping step a number of times.

When the anodized layer is stripped by this method, because currentrecovery is not carried out, the orderliness of the array of pits on thealuminum substrate side is not disturbed. Hence, this method isadvantageous in the subsequently described second embodiment of theinvention.

Examples of preferred conditions for reverse electrolysis are givenbelow.

<Preferred Conditions 1>

-   Cathode: Anodized layer obtained by anodization with aqueous    solution of oxalic acid (concentration, 0.3 mol/L; temperature, 17°    C.) at a voltage of 40 V for a treatment time of 60 minutes; layer    thickness, 60 μm.-   Anode: Carbon electrode.-   Electrolytic solution: Aqueous solution of aluminum sulfate having a    concentration of 0.04 g/L (aluminum ion basis), a pH of 3.8, an    electrical conductivity of 0.6 mS/cm, and a temperature of 33° C.-   Voltage: 40 V (voltage setting).

In the first aspect of the invention, the stripping step treatment timeis very short compared with the time required in a conventional filmremoval step involving dissolution with a mixed aqueous solution ofchromic acid and phosphoric acid. Therefore, structures can beefficiently produced by the method in the first aspect of the invention.

Moreover, in a film removal step, when the aluminum oxide content (asAl₂O₃) of a mixed aqueous solution of chromic acid and phosphoric acidexceeds 15 g/L, the solvency abruptly deteriorates, making it necessaryto replace the solution with fresh treatment solution. Because theanodized layer used in the invention generally has a large thickness,the amount of aluminum oxide which dissolves out in a single treatmentis large, resulting in rapid degradation of the treatment solution.

By contrast, in the present invention, because the anodized layer isstripped off in a solid state at the boundary with the aluminumsubstrate, it can easily be separated off with a filter or the like.Hence, the aqueous acid solution used in reverse electrolysis does notdeteriorate.

Therefore, the treatment time and the amount of aqueous acid solutionconsumed in the stripping step carried out in the invention arerespectively much shorter and much smaller than the treatment time andthe amount of treatment solution consumed in a conventional film removalstep carried out with a mixed aqueous solution of chromic acid andphosphoric acid.

In the stripping step, the barrier layer is dissolved by theabove-described reverse electrolysis, giving a structure composed of theanodized layer.

At the same time, the aluminum substrate from which the anodized layerhas been stripped becomes an aluminum substrate having a plurality ofpits. This aluminum substrate having a plurality of pits may be used inthe subsequently described second aspect of the invention. This isexplained in detail below in conjunction with the accompanying diagrams.

FIGS. 2A to 2D show diagrams illustrating the inventive method ofmanufacturing structures.

FIG. 2A is a schematic cross-sectional view of an aluminum member priorto the stripping step. As shown in FIG. 2A, an aluminum member 10 has analuminum substrate 12 and an anodized layer 14 present on the surface ofthe aluminum substrate 12. Micropores 16 are present within the anodizedlayer 14, and a barrier layer 18 is situated below the micropores 16.

FIGS. 2B and 2C are respectively schematic cross-sectional views of astructure and an aluminum substrate obtained by the stripping step.

A structure 20 shown in FIG. 2B is obtained by dissolving the barrierlayer 18 of the anodized layer 14 in the aluminum member 10 shown inFIG. 2A, and is composed of the anodized layer having pits 22.

An aluminum substrate 24 shown in FIG. 2C is obtained by dissolving thebarrier layer 18 of the anodized layer 14 in the aluminum member 10shown in FIG. 2A, and has pits 26.

The second aspect of the invention provides a method of manufacturing astructure including a stripping step in which an aluminum member thatincludes an aluminum substrate and an anodized layer present on asurface of the aluminum substrate and that serves as a cathode iselectrolyzed to strip the anodized layer from the aluminum substrate tothereby obtain a structure composed of the aluminum substrate havingpits formed therein, and an anodizing step in which the aluminumsubstrate having the pits formed therein are anodized to obtain thestructure composed of the aluminum substrate having on a surface thereofa micropore-bearing anodized layer. Electrolysis in the stripping stepis carried out in such a way that a current passes only over a surfaceof the anodized layer.

The stripping step in the second aspect of the invention is carried outin the same way as the stripping step in the first aspect of theinvention.

<Anodizing Treatment Step>

In the second aspect of the invention, an anodizing treatment step iscarried out following the stripping step.

In the anodizing treatment step, anodizing treatment is performed on thealuminum substrate having pits obtained from the stripping step, therebygiving a structure composed of the aluminum substrate having on thesurface a micropore-bearing anodized layer.

In the aluminum substrate having pits obtained from the stripping step,the shape at the bottom of the barrier layer where the micropores arethe most highly ordered is in the form of surface pits (see FIG. 2C).These surface pits are thus substantially semi-spherical and regularlyarranged like the micropores.

The anodizing treatment step may be carried out using a method known inthe art. Specifically, this step is carried out in the same way as theanodizing treatment used for obtaining the above-described aluminummember.

It is preferable to use the same type of electrolytic solution as thatused in the above-described reverse electrolysis. In this way, reverseelectrolysis and the anodizing treatment step can be carried out in thesame electrolytic bath. Moreover, even when these are carried out inseparate electrolytic baths, there are no adverse effects from thecarryover of solution into the anodizing treatment bath.

In this anodizing treatment step, the regularly arrayed pits on thesurface of the aluminum substrate serve as the starting points foranodizing treatment, leading to the formation of an anodized layerhaving an orderly array of micropores.

Therefore, the anodizing treatment step provides a structure composed ofan aluminum substrate having on the surface thereof an anodized layerwith an orderly array of micropores.

FIG. 2D is a schematic cross-sectional view of a structure obtained bythe anodized treatment step. A structure 28 shown in FIG. 2D is obtainedby subjecting the aluminum substrate 24 shown in FIG. 2C to anodizingtreatment so as to form an anodized layer 30. During anodizingtreatment, micropores 32 are formed with the pits 26 in the aluminumsubstrate 24 serving as the starting points. Therefore, the structure 28is composed of an aluminum substrate 34 having on the surface theanodized layer 30 with the micropores 32.

In the second aspect of the invention, the treatment time for thestripping step is extremely short compared with the time required for aconventional film removal step involving dissolution with a mixedaqueous solution of chromic acid and phosphoric acid. Hence, structurescan be efficiently manufactured by the method according to the secondaspect of the invention.

<Chemical Dissolution Treatment Step>

In one preferred embodiment of the second aspect of the invention,following the above-described anodizing treatment step, a chemicaldissolution treatment step is carried out in which chemical dissolutiontreatment is performed on the structure composed of an aluminumsubstrate having a micropore-bearing anodized layer on its surface. Bycarrying out the chemical dissolution treatment step, the diameter ofthe pores becomes more uniform.

Chemical dissolution treatment can be carried out in the same manner asin the above-described pore widening treatment.

<Structure>

The structure composed of an anodized layer obtained in the first aspectof the invention and the structure composed of an aluminum substratehaving a micropore-bearing anodized layer on the surface obtained in thesecond aspect of the invention both have regularly arrayed pits ormicropores, and can therefore be employed in various applications.

For example, by carrying out sealing treatment to fill the pits ormicropores with a metal, the structure can be used as a sample holderfor Raman spectroscopy.

Alternatively, the structure can be used as a nanoimprint mold.

In addition, structures composed of an anodized layer obtained accordingto the first aspect of the invention can be used as separation filters.

<Sealing Treatment>

The metal used in sealing treatment is not subject to any particularlimitation, so long as it is an element having metal bonds that includefree electrons. However, a metal in which plasmon resonance has beenrecognized is preferred. Of these, it is known that gold, silver,copper, nickel and platinum are known to readily give rise to plasmonresonance (Gendai Kagaku (Contemporary Chemistry), pp. 20-27 (September2003)), and are thus preferred. Gold and silver are especially preferredbecause of the ease of electrodeposition and colloidal particleformation.

Sealing may be carried out using any suitable known technique withoutparticular limitation.

Examples of preferred techniques include electrodeposition, and a methodwhich involves coating the structure of the present invention with adispersion of metal colloidal particles, then drying. The metal ispreferably in the form of single particles or agglomerates.

An electrodeposition method known in the art may be used. For example,in the case of gold electrodeposition, use may be made of a process inwhich the aluminum member is immersed in a 30° C. dispersion containing1 g/L of HAuCl₄ and 7 g/L of H₂SO₄ and electrodeposition is carried outat a constant voltage of 11 V (regulated with an autotransformer such asSLIDAC) for 5 to 6 minutes.

An example of the electrodeposition method which employs copper, tin andnickel is described in detail in Gendai Kagaku (Contemporary Chemistry),pp. 51-54 (January 1997)). Use can be made of this method as well.

The dispersions employed in methods which use metal colloidal particlescan be obtained by a conventionally known method. Illustrative examplesinclude methods of preparing fine particles by low-vacuum vapordeposition and methods of preparing metal colloids by reducing anaqueous solution of a metal salt.

The metal colloidal particles have an average particle size ofpreferably 1 to 200 nm, more preferably 1 to 100 nm, and even morepreferably 2 to 80 nm.

Preferred use can be made of water as the dispersion medium employed inthe dispersion. Use can also be made of a mixed solvent composed ofwater and a solvent that is miscible with water, such as an alcohol,illustrative examples of which include ethyl alcohol, n-propyl alcohol,i-propyl alcohol, 1-butyl alcohol, 2-butyl alcohol, t-butyl alcohol,methyl cellosolve and butyl cellosolve.

No particular limitation is imposed on the technique used for coatingthe anodized layer with the dispersion of metal colloidal particles.Suitable examples of such techniques include bar coating, spin coating,spray coating, curtain coating, dip coating, air knife coating, bladecoating and roll coating.

Preferred examples of dispersions that may be employed in methods whichuse metal colloidal particles include dispersions of gold colloidalparticles and dispersions of silver colloidal particles.

Dispersions of gold colloidal particles that may be used include thosedescribed in JP 2001-89140 A and JP 11-80647 A. Use can also be made ofcommercial products.

Dispersions of silver colloidal particles preferably contain particlesof silver-palladium alloys because these are not affected by the acidswhich leach out of the anodized layer. The palladium content in such acase is preferably from 5 to 30 wt %.

Application of the dispersion is followed by cleaning that may beappropriately performed using a solvent such as water. As a result ofsuch cleaning, only the particles filled into the micropores remainwhereas particles that have not been filled into the micropores areremoved.

The amount of metal deposited after sealing is preferably 100 to 500mg/m².

The surface porosity after sealing treatment is preferably not more than20%. The surface porosity after sealing treatment is defined as thetotal surface area of the openings in unsealed pits or microporesrelative to the surface area of the structure surface. When the surfaceporosity is in the above range, a stronger localized plasmon resonancecan be obtained.

At a pore diameter of 50 nm or more, it is preferable to use a sealingmethod that employs metal colloidal particles. At a pore diameter ofless than 50 nm, the use of an electrodeposition process is preferred.Suitable use can also be made of a combination of both.

In the structure that has been subjected to sealing treatment, metalseals the pits or micropores and is present on the surface of thestructure as particles.

It is generally preferable for the intervals between these metalparticles to be short so as to increase Raman enhancement. The optimalinterval is affected by the size and shape of the metal particles.Depending on the viscosity of the liquid or the molecular weight of thesubstance serving as the Raman spectroscopy sample, problems such as theinability of the sample to fully enter between the metal particles mayarise.

Accordingly, the interval between the metal particles cannot be strictlyspecified, although it is generally preferable for the interval to be ina range of 1 to 400 nm, more preferably 5 to 300 nm, and even morepreferably 10 to 200 nm. Within the above range, Raman enhancementincreases and the substance serving as the sample is generally able toenter between the metal particles.

As used herein, “metal particle interval” refers to the shortestdistance between the surfaces of neighboring particles.

<Raman Enhancement Owing to Localized Plasmon Resonance>

Raman enhancement refers to an effect in which the Raman scatteringintensity of molecules adsorbed onto the metal is enhanced by a factorof about 10⁵ to 10⁶, and is called “surface-enhanced Raman scattering”(SERS). The above-referenced publication Gendai Kagaku No. 9, pp. 20-27(2003) states that Raman enhancement can be obtained by localizedplasmon resonance using particles of metals such as gold, silver,copper, platinum and nickel.

Compared with the conventional technique, the structure that has beensubjected to sealing treatment can generate a high-intensity localizedplasmon resonance and thus, when used in Raman spectroscopy, enables astronger Raman enhancement effect to be achieved. This demonstrates theutility of sample holders for Raman spectroscopy obtained from such asealed structure.

Sample holders for Raman spectroscopy obtained from the sealed structureare used in much the same way as conventional sample holders for Ramanspectroscopy. Specifically, by irradiating with light the Ramanspectroscopy sample holder obtained from the sealed structure andmeasuring the Raman scattering intensity of the reflected or transmittedlight, the properties of a substance which is held on the sample holderand is near the metal are detected.

<Nanoprinting>

The inventive structure may be used as a nanoimprint mold. Specifically,by casting a resin or the like into the pits or micropores in theinventive structure and curing the resin, a substrate having projectionscan be obtained. This substrate having the projections can be used as,for example, an optical device.

EXAMPLES

Examples are given below by way of illustration and should not beconstrued as limiting the invention.

1. Fabrication of Structure

Examples 1 to 15, and Comparative Example 1

The substrate was subjected to, in order, mirror-like finishingtreatment, self-ordering anodizing treatment, masking and reverseelectrolysis, thereby obtaining a structure composed of an anodizedlayer, and an aluminum substrate. The aluminum substrate thus obtainedwas successively subjected to main anodizing treatment and pore wideningtreatment, giving a structure composed of the aluminum substrate.

Each treatment step is described in detail below.

(1) Substrate

The structure was manufactured using Substrate 1 below.

-   Substrate 1: High-purity aluminum. Produced by Wako Pure Chemical    Industries, Ltd. Purity, 99.99 wt %; thickness, 0.4 mm.    (2) Mirror-Like Finishing Treatment

The above substrate was subjected to the following mirror-like finishingtreatment.

<Mirror-Like Finishing Treatment>

Mirror-like finishing treatment was performed by electrolytic polishing.Electrolytic polishing was carried out for 5 minutes using anelectrolytic solution of the composition indicated below (temperature,65° C.), using the substrate as the anode and a carbon electrode as thecathode, and at a constant current of 12.5 A/dm². <Electrolytic SolutionComposition> 85 wt % Phosphoric acid (Wako Pure Chemical 1,320 mLIndustries, Ltd.) Aqueous solution of sulfuric acid (50 wt %) 600 mLPure water 20 mL(3) Self-Ordering Anodizing Treatment (Formation of Pits)

The surface of the mirror-like finished substrate was subjected toself-ordering anodizing treatment under either of the following sets ofconditions A and B, thereby forming pits. These pits served as thestarting points for micropore formation in the subsequently describedmain anodizing treatment.

<Self-Ordering Anodizing Treatment>

<Condition A>

An aqueous solution of sulfuric acid having a concentration of 0.3 mol/Land a temperature of 16° C. was prepared using sulfuric acid (a reagentproduced by Kanto Chemical Co., Inc.). The substrate (area of treatment,5 cm×10 cm) was immersed in this aqueous solution of sulfuric acid, andself-ordering anodizing treatment was carried out for 7 hours underconstant voltage conditions at a voltage of 25 V, thereby forming on thesubstrate an anodized layer having a film thickness of 90 μm.

In self-ordering anodizing treatment, use was made of a SUS304 electrodeas the cathode, NeoCool BD36 (Yamato Scientific Co., Ltd.) as thecooling system, Pairstirrer PS-100 (Tokyo Rikakikai Co., Ltd.) as thestirring and warming unit, and a GP0650-2R unit (Takasago, Ltd.) as thepower supply. PET tape (DANPRON Tape, produced by Nitto DenkoCorporation) was affixed beforehand to the side of the substrate notfacing the electrode surface so as to keep it from being anodized.

<Condition B>

An aqueous solution of oxalic acid having a concentration of 0.5 mol/Land a temperature of 16° C. was prepared using oxalic acid dihydrate (areagent produced by Kanto Chemical Co., Inc.). The substrate (area oftreatment, 5 cm×10 cm) was immersed in this aqueous solution of oxalicacid, and self-ordering anodizing treatment was carried out for 5 hoursunder constant voltage conditions at a voltage of 40 V, thereby formingon the substrate an anodized layer having a film thickness of 45 μm.

In self-ordering anodizing treatment, use was made of a SUS304 electrodeas the cathode, NeoCool BD36 (Yamato Scientific Co., Ltd.) as thecooling system, Pairstirrer PS-100 (Tokyo Rikakikai Co., Ltd.) as thestirring and warming unit, and a GP0650-2R unit (Takasago, Ltd.) as thepower supply. PET tape (DANPRON Tape, produced by Nitto DenkoCorporation) was affixed beforehand to the side of the substrate notfacing the electrode surface so as to keep it from being anodized.

<Film Thickness Measurement Method>

The substrate on which an anodized layer had been formed was bent, theedge (fracture plane) in a portion of the specimen where crackingoccurred was examined with an ultrahigh-resolution scanning electronmicroscope (Hitachi S-900, manufactured by Hitachi, Ltd.) at anacceleration voltage of 20 V and a magnification of 200×, and the filmthickness was measured. Ten spots were randomly selected on eachspecimen, and the average value of the measurements was used as the filmthickness. The film thickness value at each of the ten spots was withina range of the average value ±10%.

(4) Masking

Masking was carried out on the substrate that had been subjected toself-ordering anodizing treatment. The masking method is shown inTable 1. Notations used in Table 1 are explained below.

-   “None”: After self-ordering anodizing treatment, the PET tape was    peeled off and the substrate was subjected to reverse electrolysis    without carrying out masking.-   “Back”: After self-ordering anodizing treatment, the substrate was    subjected to reverse electrolysis with the PET tape left in place.-   “Back+Edges”: After self-ordering anodizing treatment, the PET tape    was left in place. In addition, the edges of the substrate were    coated with a two-component mixed epoxy resin (Araldite, available    from Nichiban Co., Ltd.) and the substrate was left to stand for one    day to allow the resin to cure, then was subjected to reverse    electrolysis.-   “Back+Edges+Surface A”: After the above-described “Back+Edges”    treatment, PET tape (DANPRON Tape, produced by Nitto Denko    Corporation) having a circular opening of 1.7 cm radius was affixed    to the surface on the anodized layer side of the substrate. The PET    tape was affixed so that the opening was positioned at substantially    the center of the substrate and the cured resin on the edges of the    substrate was also covered.-   “Back+Edges+Surface B”: After the above-described “Back+Edges”    treatment, PET tape (DANPRON Tape, produced by Nitto Denko    Corporation) having an elliptical opening with a major axis of 2.5    cm and a minor axis of 1.4 cm was affixed to the surface on the    anodized layer side of the substrate. The PET tape was affixed so    that the opening was positioned at substantially the center of the    substrate and the cured resin on the edges of the substrate was also    covered.-   “Back+Edges+Surface C”: After the above-described “Back+Edges”    treatment, PET tape (DANPRON Tape, produced by Nitto Denko    Corporation) having an opening in the form of a square measuring 3    cm on a side with corners that are rounded to a radius of 5 mm was    affixed to the surface on the anodized layer side of the substrate.    The PET tape was affixed so that the opening was positioned at    substantially the center of the substrate and the cured resin on the    edges of the substrate was also covered.-   “Back+Edges+Surface D”: After the above-described “Back+Edges”    treatment, PET tape (DANPRON Tape, produced by Nitto Denko    Corporation) having an opening in the form of a square measuring 3    cm on a side was affixed to the surface on the anodized layer side    of the substrate. The PET tape was affixed so that the opening was    positioned at substantially the center of the substrate and the    cured resin on the edges of the substrate was also covered.-   “Special Jig”: A special jig was positioned as shown in FIG. 3 on    the anodized layer 14 side of the aluminum substrate 12 (aluminum    member 10) on which the anodized layer 14 had been formed, and the    substrate was subjected to reverse electrolysis. The special jig had    a cathode 40, a packing 42 with an inner space that accommodates the    cathode 40, and an electrolytic solution inlet 44 and an    electrolytic solution outlet 46, both provided in the packing 42.    The cathode 40 had a diameter of 3 cm. The inner space of the    packing 42 had a diameter of 3 cm and a depth of 2 cm. A check valve    (not shown) which also served as a pressure valve was provided in    the electrolytic solution outlet 46. During reverse electrolysis,    the electrolytic solution (indicated by hatching in FIG. 3) flowed    through the electrolytic solution inlet 44 into the inner space of    the packing 42, and flowed out from the inner space of the packing    42 through the electrolytic solution outlet 46. Moreover, the    packing 42 was pressed against the anodized layer 14 so that the    edges of the packing 42 were in a state of close contact with the    anodized layer 14 to prevent the electrolytic solution from leaking.    FIG. 3 is a schematic view illustrating a special jig that may be    employed in reverse electrolysis. Details such as the size of the    micropores 16 relative to the size of the special jig differ from    reality.    (5) Reverse Electrolysis

Following masking, reverse electrolysis was carried out, using theabove-described substrate on which an anodized layer had been formed asthe cathode and a platinum electrode as the anode, in an aqueoussolution of aluminum sulfate having a concentration of 4.5 g/L and atemperature of 33° C. and under constant voltage conditions at a voltageof 16 V. The substrate on which the anodized layer had been formed wasplaced in an aqueous solution of aluminum sulfate with the planardirection of the substrate oriented vertically. The anodized layer wasthus stripped from the aluminum substrate.

In reverse electrolysis, monitoring of the current value was carriedout. Table 1 shows the maximum current value, the current value at theend of reverse electrolysis, and the current ratio (current value at theend of reverse electrolysis/maximum current value).

In Example 15, instead of constant voltage conditions, reverseelectrolysis was carried out by gradually increasing the voltage from 0V to 17.8 V at a rate of 2 V/min in a linear manner (in Table 1, amaximum current value and a current ratio are not shown for thisexample).

Following the completion of reverse electrolysis, PET tape (DANPRONTape, produced by Nitto Denko Corporation) was affixed to the surface ofthe anodized layer side. The boundary between the region which came intocontact with the electrolytic solution and the region which did not comeinto contact with the electrolytic solution was then scored with acutter, and the anodized layer was separated from the aluminumsubstrate.

(6) Main Anodizing Treatment

The aluminum substrate obtained by stripping off the anodized layer inreverse electrolysis was subjected to a main anodizing treatment. Asidefrom setting the treatment time to 2 minutes, the main anodizingtreatment was carried out under the same set of conditions A or B aswere used in self-ordering anodizing treatment.

(7) Pore Widening Treatment

The aluminum substrate obtained following the main anodizing treatmentwas subjected to pore widening treatment to enhance the uniformity ofthe subsequently described sealing treatment. Pore widening treatmentwas carried out by immersing the aluminum substrate for 15 minutes in anaqueous solution of phosphoric acid having a concentration of 5 wt %(temperature, 30° C.).

2. Evaluation of Stripped State

The state of separation between the anodized layer and the aluminumsubstrate following reverse electrolysis was evaluated in each of theabove examples.

That is, the stripped surface of the aluminum substrate followingreverse electrolysis was visually examined. By inking in with a pen anywhite, darkened areas that lacked specular gloss and carrying out imageanalysis, the area ratio of the region having specular gloss wascomputed, based on which the uniformity of stripping was assessed.

The results are shown in Table 1. In the table, the “Strippinguniformity” was rated as A when the area ratio of regions having aspecular gloss was more than 95% and up to 100%, as B when the arearatio of regions having a specular gloss was more than 90% and up to95%, as C when the area ratio of regions having a specular gloss wasmore than 85% and up to 90%, as D when the area ratio of regions havinga specular gloss was more than 80% and up to 85%, as E when the arearatio of regions having a specular gloss was more than 70% and up to80%, and as F when the area ratio of regions having a specular gloss wasup to 70%.

3. Evaluation of Structure Composed of Aluminum Substrate

The degree of ordering, which is an indicator of the regularity of themicropores, was determined for the aluminum substrate following the mainanodizing treatment. The results are shown in Table 1.

The degree of ordering is defined by the following formula (1).Degree of Ordering(%)=B/A×100  (1)

In Formula (1), A represents the total number of micropores in ameasurement region; and B represents the number of specific microporesin the measurement region for which, when a circle is drawn so as to becentered on the center (of gravity) of a specific micropore and so as tobe of the smallest radius that is internally tangent to the edge ofanother micropore, the circle includes the centers (of gravity) of sixmicropores other than the specific micropore.

FIGS. 4A and 4B show diagrams illustrating the method for computing thedegree of ordering of the pores. Formula (I) is explained more fullybelow in conjunction with FIGS. 4A and 4B.

With regard to a micropore 1 shown in FIG. 4A, when a circle 3 is drawnso as to be centered on the center (of gravity) of the micropore 1 andso as to be of the smallest radius that is internally tangent to theedge of another micropore (inscribed in a micropore 2), the interior ofthe circle 3 includes the centers (of gravity) of six micropores otherthan the micropore 1. Therefore, the micropore 1 is included in B.

With regard to a micropore 4 shown in FIG. 4B, when a circle 6 is drawnso as to be centered on the center (of gravity) of the micropore 4 andso as to be of the smallest radius that is internally tangent to theedge of another micropore (inscribed in a micropore 5), the interior ofthe circle 6 includes the centers (of gravity) of five micropores otherthan the micropore 4. Therefore, the micropore 4 is not included in B.With regard to a micropore 7 shown in FIG. 4B, when a circle 9 is drawnso as to be centered on the center (of gravity) of the micropore 7 andso as to be of the smallest radius that is internally tangent to theedge of another micropore (inscribed in a micropore 8), the interior ofthe circle 9 includes the centers (of gravity) of seven micropores otherthan the micropore 7. Therefore, the micropore 7 is not included in B.TABLE 1 Current Reverse Maximum value at end Self-ordering electrolysiscurrent of reverse Current Degree of anodizing voltage valueelectrolysis ratio Stripping ordering condition Masking method (V)(mA/dm²) (mA/dm²) (%) uniformity (%) Comp. Ex. 1 A None 25 500 500 100 F— Example 1 A Back 25 300 200 67 E 50 Example 2 A Back + Edges 25 250 6024 C 50 Example 3 A Back + Edges 25 250 100 40 D 50 Example 4 A Back +Edges 25 250 150 60 E 50 Example 5 A Back + Edges + Surface A 25 250 6024 A 70 Example 6 A Back + Edges + Surface B 25 250 60 24 A 60 Example 7A Back + Edges + Surface C 25 250 60 24 B 50 Example 8 A Back + Edges +Surface D 25 250 60 24 C 50 Example 9 A Back + Edges + Surface A 20 25020 8 C 50 Example 10 A Back + Edges + Surface A 18 250 20 8 B 60 Example11 A Back + Edges + Surface A 15 250 30 12 A 70 Example 12 A Back +Edges + Surface A 13 250 50 20 E 60 Example 13 B Back + Edges + SurfaceA 40 250 60 24 A 70 Example 14 A Special Jig 25 250 60 24 A 70 Example15 A Back + Edges + Surface A 0 → 17.8 — 30 — A 50

As is apparent from Table 1, the inventive methods of manufacturingstructures (Examples 1 to 15) provided in each case an excellentstripping uniformity, and the structures composed of an aluminumsubstrate thus obtained all had well-ordered arrays of pores.

By contrast, when reverse electrolysis was carried out in such a waythat the current passed through the aluminum substrate (ComparativeExample 1), the stripping uniformity was poor.

1. A method of manufacturing a structure comprising: a stripping step inwhich an aluminum member that includes an aluminum substrate and ananodized layer present on a surface of the aluminum substrate and thatserves as a cathode is electrolyzed to strip the anodized layer from thealuminum substrate to thereby obtain a structure composed of theanodized layer, wherein electrolysis in the stripping step is carriedout in such a way that a current passes over a surface of the anodizedlayer.
 2. The method of manufacturing the structure according to claim1, wherein the electrolysis in the stripping step is carried out in sucha way that the current passes only over the surface of the anodizedlayer.
 3. The method of manufacturing the structure according to claim2, wherein the electrolysis in the stripping step is carried out in astate where an electrolytic solution is in contact with the surface ofthe anodized layer but is in contact with neither the aluminum substratenor edges of the anodized layer.
 4. The method of manufacturing thestructure according to any one of claims 1 to 3, wherein theelectrolysis in the stripping step reaches completion when the currentfalls to a value of 0.1 A/dm² or below.
 5. A structure obtained by themethod according to any one of claims 1 to
 4. 6. A method ofmanufacturing a structure comprising: a stripping step in which analuminum member that includes an aluminum substrate and an anodizedlayer present on a surface of the aluminum substrate and that serves asa cathode is electrolyzed to strip the anodized layer from the aluminumsubstrate to thereby obtain a structure composed of the aluminumsubstrate having pits formed therein; and an anodizing step in which thealuminum substrate having the pits formed therein are anodized to obtainthe structure composed of the aluminum substrate having on a surfacethereof a micropore-bearing anodized layer, wherein electrolysis in thestripping step is carried out in such a way that a current passes over asurface of the anodized layer.
 7. The method of manufacturing thestructure according to claim 6, wherein the electrolysis in thestripping step is carried out in such a way that the current passes onlyover the surface of the anodized layer.
 8. The method of manufacturingthe structure according to claim 7, wherein the electrolysis in thestripping step is carried out in a state where an electrolytic solutionis in contact with the surface of the anodized layer but is in contactwith neither the aluminum substrate nor edges of the anodized layer. 9.The method of manufacturing the structure according to any one of claims6 to 8, wherein the electrolysis in the stripping step reachescompletion when the current falls to a value of 0.1 A/dm² or below. 10.The method of manufacturing the structure according to any one of claim6 to 9, further comprising a chemical dissolution treatment step whichfollows the anodizing step and in which the structure composed of thealuminum substrate having on the surface thereof the micropore-bearinganodized layer is subjected to a chemical dissolution treatment.
 11. Astructure obtained by the method according to any one of claims 6 to 10.